Originally called project metis, this feature allows you to do a lot of cool stuff, such as upgrading to a x265 encode, downloading releases with multiple languages, etc. Check out the wiki page at: https://github.com/Radarr/Radarr/wiki/Custom-Formats to learn more! Note: This feature is currently in "beta" and will get more tags and features in the future. Please let me know, if you have any issues and I hope this will allow for a lot of customization!
Leonardo Galli